The functional data presented in this application indicate that an H+/K+-ATPase plays a major role in urinary acidification by the inner stripe of the outer medullary collecting duct (OMCDi) during normal (K-replete) as well as K-restricted circumstances. The structural evidence indicates that an H+/K+-ATPase is present in both intercalated cells and principal cells of the OMCDi and in inner medullary collecting duct (IMCD) cells. Further evidence suggests that more than one (a subunit) isoform of H+/K+-ATPase is present in the kidney. Accordingly, the applicant hypothesizes: 1) renal H+/K+-ATPase is a major proton pump that is responsible for luminal acidification by the OMCDi; 2) renal H+/K+-ATPase activity, defined biochemically, represents at least two isoforms for the a subunit; and 3) renal H+/K+-ATPase is responsible for luminal acidification by principal cells of the OMCDi and by IMCD cells of the IMCD as well as by intercalated cells of the collecting duct. Thus, the applicant formulates the following three Specific Aims: 1) to characterize in detail the mechanism of luminal acidification and associated apical K channels in the OMCDi. A series of microperfusion and patch-clamp experiments are proposed that will examine in detail the mechanism of luminal acidification in the OMCDi and the ion channels present at the apical membrane of the OMCDi, respectively; 2) to determine the alpha and beta isoforms of H+/K+-ATPase present in the OMCDi. A strategy that has identified a novel renal H,OMCDi-ATPase a isoform is proposed to determine H+/K+-ATPase alpha and alpha subunit isoforms of the OMCDi; 3) to determine the cellular distribution of H+/K+-ATPase alpha and beta subunit mRNAs and proteins in the kidney. A series of studies utilizing in situ hybridization and immunohistochemistry will examine the distribution of H+/K+-ATPase subunit mRNA and protein within the kidney. Specifically, the localization in principal cells and intercalated cells of the OMCDi and in IMCD cells of the IMCD will be examined. This integrative functional and structural approach has yielded important new information regarding the basic mechanism of operation of H+/K+-ATPase, the isoforms of the catalytic subunit present in the kidney, and their distribution. Accordingly, these studies represent a systemic examination of renal H+/K+-ATPase and have opened important new areas of investigation. In particular, these experiments should provide insight into the fundamental mechanism of proton secretion by H+/K+-ATPase in general and, as such, should have significant implications beyond luminal acidification by the OMCDi.